UNIVERSITI TEKNIKAL MALAYSIA MELAKA

IMPROVEMENT OF TRIBOLOGICAL PROPERTIES OF ZINC

DIALKYLDITHIOPHOSPHATE (ZDDP) INDUCED CORN OIL

BIO-LUBRICANT WITH THE ADDITION OF MOLYBDENUM

DIALKYLDITHIOPHOSPHATE (MoDTP)

This report is submitted in accordance with the requirement of Universiti Teknikal

Malaysia Melaka (UTeM) for the Bachelor of Mechanical Engineering Technology

(Maintenance Technology) with Honours

by

LOKMAN HAKIM BIN MAT

B071310586

940122-06-5751

FACULTY OF ENGINEERING TECHNOLOGY

2016

UNIVERSITI TEKNIKAL MALAYSIA MELAKA

BORANG PENGESAHAN STATUS LAPORAN PROJEK SARJANA MUDA

TAJUK: IMPROVEMENT OF TRIBOLOGICAL PROPERTIES OF ZINC

DIALKYLDITHIOPHOSPHATE (ZDDP) INDUCED CORN OIL BIO-LUBRICANT WITH THE

ADDITION OF MOLYBDENUM DIALKYLDITHIOPHOSPHATE (MoDTP)

SESI PENGAJIAN: 2016/17 Semester 1

Saya LOKMAN HAKIM BIN MAT

mengaku membenarkan Laporan PSM ini disimpan di Perpustakaan Universiti Teknikal

Malaysia Melaka (UTeM) dengan syarat-syarat kegunaan seperti berikut:

1. Laporan PSM adalah hak milik Universiti Teknikal Malaysia Melaka dan penulis. 2. Perpustakaan Universiti Teknikal Malaysia Melaka dibenarkan membuat salinan untuk

tujuan pengajian sahaja dengan izin penulis. 3. Perpustakaan dibenarkan membuat salinan laporan PSM ini sebagai bahan pertukaran

antara institusi pengajian tinggi.

4. **Sila tandakan ( )

SULIT

TERHAD

TIDAK TERHAD

(Mengandungi maklumat yang berdarjah keselamatan

atau kepentingan Malaysia sebagaimana yang termaktub

dalam AKTA RAHSIA RASMI 1972)

(Mengandungi maklumat TERHAD yang telah ditentukan

oleh organisasi/badan di mana penyelidikan dijalankan)

Alamat Tetap:

NO.32,

PAKR SUNGAI KERPAN 3,

27200 KUALA LIPIS,

PAHANG DARUL MAKMUR.

Tarikh: ________________________

Disahkan oleh:

Cop Rasmi:

Tarikh: _______________________

** Jika Laporan PSM ini SULIT atau TERHAD, sila lampirkan surat daripada pihak berkuasa/organisasi

berkenaan dengan menyatakan sekali sebab dan tempoh laporan PSM ini perlu dikelaskan sebagai

SULIT atau TERHAD.

DECLARATION

I hereby, declared this report entitled “Improvement of Tribological Properties of

Zinc Dialkyldithiophosphate (ZDDP) Induced Corn Oil Bio-Lubricant with the

Addition of Molybdenum Dialkyldithiophosphate (MoDTP)” is the results of my

own research except as cited in references.

Signature : …………………………………

Name : LOKMAN HAKIM BIN MAT

Date : 16 JANUARY 2017

APPROVAL

This report is submitted to the Faculty of Engineering Technology of UTeM as a

partial fulfillment of the requirements for the degree of Bachelor of Engineering

Technology (Maintenance Techonology) (Hons.). The member of the

supervisory is as follow:

…………………………………

(Muhamad Azwar Bin Azhari)

i

ABSTRAK

Permintaan untuk pelincir alternatif dan kebimbangan kepada isu-isu alam sekitar

mendorong kajian ini untuk membangunkan bio-pelincir baru berasaskan minyak

sayur-sayuran. Walau bagaimanapun, prestasi minyak sayuran sebagai minyak

pelincir tidak berkesan kerana menghasilkan geseran yang tinggi pada permukaan.

Dalam kajian ini, minyak jagung komersial dicampurkan dengan bahan tambahan

Zink dialkyldithiophosphate (ZDDP) dan Molybdenum dialkyldithiophosphate

(MoDTP) untuk meningkatkan sifat-sifat fizikal dan tribologikal minyak. Minyak

masak jagung komersial telah dipilih sebagai minyak asas dan dicampur dengan 2%

berat ZDDP dan ditambah dengan 0.00% berat, 0.05% berat, 0.10% berat, 0.15%

berat dan 0.20% berat MoDTP. Sampel diuji menggunakan Elektrod Cakera

Berputar-Spektroskopi Atom Terpancar (RDE-AES) dan Meter Kelikatan Terpanas

Kittiwake manakala dicirikan menggunakan Penguji Empat Bola dan Mikroskop

Cahaya Sejajar untuk menyiasat pekali geseran dan diameter parut kehausan.

Penambahan 2% berat ZDDP dan 0.05% berat MoDTP ke dalam minyak jagung

memberikan hasil yang diingini di mana nilai kelikatan kinematik pada 36.20 cSt dan

pekali terendah geseran pada 0.074. Campuran ini juga menunjukkan diameter parut

kehausan yang rendah pada 74.35 µm berbanding sampel lain. Kesimpulannya, bio-

minyak pelincir yang baru telah berjaya dibangunkan dengan menambah ZDDP dan

MoDTP ke dalam minyak jagung sebagai pengganti kepada komersial minyak

pelincir.

ii

ABSTRACT

The demand for an alternative lubricant and concern to the environmental issues

prompted this study to develop a new bio-lubricant based on vegetable oil. However,

the performance of vegetable oil as lubricant oil is ineffective because of producing

high friction on the surface. In this study, the commercialized corn oil was mixed

with additives of Zinc dialkyldithiophosphate (ZDDP) and Molybdenum

dialkyldithiophosphate (MoDTP) as to improve the physical and tribological

properties of oil. The commercial cooking corn oil was selected as the base oil and

mixed with 2wt% ZDDP and added with 0.00wt%, 0.05wt%, 0.10wt%, 0.15wt% and

0.20wt% MoDTP. The samples were tested using Rotating Disc Electrode Atomic

Emission Spectroscopy (RDE-AES) and Kittiwake Heated Viscometer while

characterize using Four Ball Tester and Upright Light Microscope to investigate the

coefficient of friction and wear scar diameter. The addition of 2wt% ZDDP and

0.05wt% MoDTP into corn oil gives the desired result where the kinematic viscosity

value at 36.20 cSt and lowest coefficient of friction at 0.074. This mixture also

demonstrated lowest wear scar diameter at 74.35 µm compared to the other samples.

In conclusion, the newly bio-lubricant has successfully developed by adding ZDDP

and MoDTP into corn oil as a substitute for commercial mineral lubricant oil.

iii

DEDICATION

I would like to dedicate my thesis to my beloved parents, siblings and friends.

iv

ACKNOWLEDGMENT

I would like to express my gratitude and appreciation to the most Merciful and

Almighty, ALLAH SWT for giving His bless upon completing this final year project

report successfully.

In addition, I also want to address my deepest appreciation to Mr. Muhamad

Azwar Azhari for his guidance, motivation and full commitment by helping in

completing my research and writing of this Final Year Project.

Plus, my sincere thanks to all my friends that have spent their time in helping

me during my research and writing and last but not least, I would like to express my

biggest gratitude towards my family especially my parent, Mat Jaafar and Salmah

Muhamad Ali, for their support in moral and physically. Thank you.

v

TABLE OF CONTENT

PAGE

Declaration

Approval

Abstrak i

Abstract II

Dedication III

Acknowledgment IV

Table of Content V

List of Tables VIII

List of Figures IX

List of Symbols and Abbreviations X

CHAPTER 1: INTRODUCTION 1

1.1 Introduction to Lubricant 1

1.2 Problem Statement 2

1.3 Objective of Research 5

1.4 Scope of Research 6

CHAPTER 2: LITERATURE REVIEW 7

2.1 Lubrication 7

2.2 Classification of Lubricant 8

2.2.1 Solid Lubricant 8

2.2.2 Semi-solid Lubricant 10

2.2.3 Liquid Lubricant 11

2.3 Lubrication Regime 13

2.3.1 Hydrodynamic Lubrication 13

2.3.2 Boundary Lubrication 14

2.3.3 Mixed Lubrication 16

2.4 Mineral Based Oil as Lubricant 16

2.5 Synthetic Oil as Lubricant 17

i

ii

iii

iv

v

viii

ix

x

vi

2.6 Vegetable Oil as Lubricant 18

2.6.1 Corn Oil 19

2.6.2 Canola Oil 20

2.6.3 Palm Oil 21

2.7 Additive Response in Vegetable Oil 21

2.7.1 Anti-Oxidant 22

2.7.2 Anti-Wear 23

2.7.3 Friction Modifier 24

2.8 Additives in Lubricant 25

2.8.1 ZDDP 26

2.8.2 MoDTP 27

CHAPTER 3: METHODOLOGY 32

3.1 Research Design 32

3.2 Material Selection 33

3.2.1 Corn Oil 33

3.2.2 Zinc Dialkyl Dithiophosphate (ZDDP) 34

3.2.3 Molybdenum Dialkyl Dithiophosphate (MoDTP) 34

3.3 Sample Preparation 35

3.3.1 Blending Method 35

3.4 Sample Testing 36

3.4.1 Metal Content Determination (ASTM D6595) 36

3.4.2 Kinematic Viscosity 37

3.5 Sample Characterization 38

3.5.1 Coefficient of Friction 38

3.5.2 Wear Scar Diameter 39

3.5.3 Upright Light Microscope 40

CHAPTER 4: RESULT AND DISCUSSION 41

4.1 Formulation of a New Bio-Lubricant 41

4.2 Determination of Metal Content in New Bio-Lubricant 42

4.3 Kinematic Viscosity of Corn Oil based on Concentration of MoDTP 46

4.4 Coefficient of Friction of Corn Oil based on Concentration of MoDTP 48

vii

4.5 Wear Scar Diameter of Corn Oil based on Concentration of MoDTP 51

CHAPTER 5: CONCLUSION AND FUTURE WORKS 55

5.1 Conclusion 55

5.2 Recommendation 56

REFERENCES 57

viii

LIST OF TABLES

TABLE TITLE PAGE

2.1 Type of liquid lubricants (Bhushan, 2013) 12

2.2 Autoxidation of lubricating oil (Azhari et al., 2014) 23

2.3 Literature mapping of antioxidant in vegetable oil based on

researchers

28

3.1 Fatty Acid Composition in Corn Oil 33

3.2 Concentration ZDDP and MoDTP into Corn Oil

36

4.1 The weight of ZDDP and MoDTP into Corn Oil 42

4.2 The amount of metal content in the sample 43

4.3 Kinematic Viscosity of Corn Oil with Different Concentration of

MoDTP

46

4.4 Coefficient of Friction of Corn Oil Added With Different

Concentration of MoDTP

48

4.5 Wear Scar Diameter of Corn Oil with Addition of Different

Concentration of MoDTP

51

ix

LIST OF FIGURES

FIGURE TITLE PAGE

2.1 Differences between liquid lubricant and solid lubricant

(Rudnick, 2009).

9

3.1 Flow Chart for Methodology 32

3.2 Structure of Zinc Dialkyl Dithiophosphate 34

3.3 Structure of Molybdenum Dialkyl Dithiophosphate 35

3.4 Rotating Disc Electrode Atomic Emission Spectroscopy

Structure (RDE-AES)

37

3.5 Structure of Kittiwake Heated Viscometer 38

3.6 Structure of Four-Ball Tester

39

4.1 Metal content in corn oil with addition of MoDTP and

ZDDP

43

4.2 Metal content in corn oil with addition of MoDTP and

ZDDP

46

4.3 Coefficient of Friction of Corn Oil with Different

Concentration MoDTP

49

4.4 Wear Scar Diameter of Corn Oil Added with Different

Concentration of MoDTP

51

4.5 Images of wear scar diameter on the steel ball surface by

using upright light microscope.

54

x

LIST OF SYMBOLS AND ABBREVIATIONS

+ - Plus

ASTM - American Society for Testing and Materials

℃ - Degree Celcius

cSt - Centistokes

g - gram

HO• - Hydroxy Radical

HP - Hindered Phenolic

mm - millimeter

MoDTC - Molybdenum DialkylDithiocarbamate

MoDTP - Molybdenum DialkylDithiophosphate

MoS2 - Molybdenum Disulfide

N - Newton

NNDM - N, N-bis (2-hydroxyethyl)-dodecanamide molybdate

PAO - Polyalphaolefin

PKO - Palm Kernel Oil

PTFE - Polytetrafluoroethylene

R• - Free Radical

RDE-AES - Rotating Disc Electron Atomic Emission Spectroscopy

RO• - Alkoxy Radical

ROO• - Peroxy Radical

ROOH - Hydroperoxides

SAE - Society of Automotive Engineers

WSD - Wear Scar Diameter

wt - Weight

ZDDC - Zinc DiamylDithiocarbamate

ZDDP - Zinc DialkylDithiophosphate

xi

μm - Micrometer

% - Percent

nm - Nanometer

rpm - Revolutions per Minute

psi - Pounds per Square Inch

RBD - Refined, Bleached and Deodorized

1

CHAPTER 1

INTRODUCTION

1.1 Introduction to Lubricant

The main purpose of lubrication is to reduce wear, heat loss and the

coefficient of friction between two contacting surfaces. Besides, lubrication is used

to prevent rust and reduce oxidation process. Lubrication also can act as an insulator

in transformer applications and formed a boundary layer against dirt, dust and water

(Mobarak et al., 2014). A study by Ahmed and Nassar, (2011) stated that the main

function of the engine oil lubricant is to prolong the lifetime of the equipment under

different kind of conditions which are speed, pressure and temperature. The lubricant

flow sufficiently at low temperature to ensure that the moving parts are not famished

of oil and help to minimize the wear by apart the moving parts at high temperature.

Lubrication occurs when two surfaces are separated by a lubricant film and a

good lubricant has the characteristics of high viscosity index, thermal stability, high

boiling point, low freezing point, corrosion prevention capability and high resistance

to oxidation (Mobarak et al., 2014). The automotive engines without effective

lubrication will reduce efficiency of fuel and shorten the life of equipment due to

high friction and excessive wear. To solve this problem, the proper selected oil with

efficient additive must be applied on the interacting metal surface (Komvopoulos and

Pernama, 2006). Lubrication generally reduces friction between moving surface by

substitute the fluid friction for mechanical friction. Other than that, lubrication

functions as liquid sealing, contaminant suspension, corrosion protection and heat

transfer. Crucial to know that the wear and heat can be reduced to the acceptable

level but they cannot be eliminated completely (Bilal et al., 2013).

2

Mobarak et al. (2014) stated that lubricants can be classified based on their

physical appearances which are solid, semi solid and liquid. Solid is when the

composition of inorganic or organic compounds such as molybdenum disulphide

forms the film of a solid material. Next, semi solid is the liquid suspended in a

thickener or additives of solid matrix, as example is grease. Lastly, the lubricant in

liquid form is petroleum oil, vegetable oil, synthetic oil and animal oil.

In contrast, lubricants also can be classified into two categories regarding to

their sources which are mineral oil lubricant and the bio-lubricant. The major

concern is the using of petroleum based lubricants in industries that can causes to

environment pollution (Mahipal et al., 2014). Mineral oil based lubricant is produce

from crude oil sources and it can be harmful to the environment (Bilal et al., 2013).

Other than that, the depletion of crude oil, the increasing of oil prices and the

pollution defects the environment have led the interest to develop and using

alternative lubricants (Mobarak et al., 2014). Due to concern of environment,

lubricants are synthesized from plant oils and other environmentally friendly sources

known as bio-lubricant which is derived from plants and animals (Bilal et al., 2013).

A research by Mobarak et al., (2014) stated that bio-lubricants have high

flash point, high lubricity, high viscosity index, and low evaporative losses. The

popular bio-lubricant is from various parent oil such as soybean, cottonseed,

sunflower, peanut, palm, coconut, castor and corn oils (Azhari et al., 2014).

Vegetable oil is believed to play an important role to substitute the mineral based oil

because it has many advantages over base lubricant which are renewability,

biodegradability and less toxicity (Shahabuddin et al., 2013).

1.2 Problem Statement

The reserve crude oil in the world is undergoing depletion, while the oil

prices increasing and the demand to protect environment from pollution have led to

develop and using alternative lubricant (Mobarak et al., 2014). Most of the available

lubricants are based on mineral oil derived from petroleum oil but it is not adaptable

to the environment because containing toxicity and non-biodegradable sources

3

(Shahabuddin et al., 2013). According to Bilal et al. (2013) lubricant based mineral

oil gained from crude oil can be harmful to the environment laterally with human life

extended. Mineral oil based lubricant are used in industries always be a concern to

the environmental pollution (Mahipal et al., 2013).

Several environmental hazards occur from the wasted oil produced and

subsequent disposal which are found in the hydraulic, agriculture, mining and

petrochemical industries (Mahipal et al., 2013). Besides, Srivastava and Sahai (2013)

reported that lubricant losses to the environment by the action of evaporation, spills

and leakages have become the major concerns to the issue of pollution and

environmental health. Annually, there are 12 million tons of lubricant waste was

released to the environment (Shahabuddin et al., 2013). According to Srivastava and

Sahai (2013) reported that every year about 5 to 10 million tons of petroleum

products was released to the environment and 40% of them are from the urban

runoff, industrial and municipal waste, refinery processes and condensation from

marine engine exhaust. Moreover, the wasted of mineral oil based lubricants are not

easy to dispose due to its non-biodegradable nature (Shahabuddin et al., 2013). This

problem can be overcome by using the sources from vegetable oil because of its

renewability and biodegradability characteristics.

Vegetable oil is one of sources for producing alternative lubricant because it

has advantages of renewable sources, environmentally friendly, less toxicity,

biodegradability and more (Shahabuddin et al., 2013). However, a study by Bilal et

al. (2013) stated that vegetable oil based lubricant has some disadvantages which are

high viscosity at low temperature and poor oxidative stability at high temperature

makes it easily to oxidize. Without additive, vegetable oil based lubricant cannot

reduce friction and wear effectively. To overcome these problems, additives are

designed specifically for plant based lubricants to eliminate the problem related to

low and high temperatures. A research by Azhari et al. (2015a) proved that Zinc

Dialkyldithiophosphate (ZDDP) is the solution to overcome the oxidation problem in

the vegetable oil based lubricants. The addition of ZDDP into vegetable oil shows

that it can improve the physical properties compared to vegetable oil without ZDDP

and give a better performance in kinematic viscosity, wear scar diameter and

coefficient of friction.

4

Basically, ZDDP reacted to the asperities and formed a thin oil film to

separate between two surfaces and reduce contact (Rudnick, 2009). In contrast, from

the prior study by Azhari et al., (2015a) shows that the different wear behaviour will

occur if the concentration is too much. The higher concentration will lead to the

increases the thickness of film and decreasing the antiwear qualities. The 2wt%

concentration of ZDDP into canola, karanja and corn oil provided the better

performance based on coefficient of friction, wear scar diameter and kinematic

viscosity compare than the others concentration. The blending of ZDDP into corn oil

has proved to be a good bio-lubricant according to excellent performance in reducing

coefficient of friction and wear (Azhari et al., 2015a). The 2wt% concentration of

ZDDP also provided an excellent performance by combining with the karanja oil

(Mahipal et al., 2014). Further investigation will be developing by adding

molybdenum organic metallic compound which is Molybdenum

Dialkyldithiophosphate (MoDTP) into ZDDP induced corn oil bio-lubricant for

improving the tribological properties.

In the group of metal dithiocarbamates, Molybdenum metallic compounds

have the most interest in the engine crankcase lubricants (Rudnick, 2009).

Molybdenum compound has been proved as friction modifiers for several decades

because the properties of antifriction, antioxidant, antiwear and extreme temperature

(Unnikrishnan et al., 2002). On the other note, it has been proven as friction modifier

additive for engine oil by formulated Molybdenum disulfide (MoS2) through

complex tribochemical reactions which is known to reduce friction (Komvopoulos

and Pernama, 2006). The decomposition of molybdenum compound are weakly

linked by Van Der Waals forces and it is easily broke its bond to be excellent friction

performance (Liskiewicz et al., 2013). However, molybdenum compound cannot be

used as an additive alone because its solubility in base oil is low. This problem

causes the preventing of using molybdenum compound in many commercial

formulations.

The solution to overcome this problem is by combination with the antiwear

agent such as ZDDP into molybdenum compound (Unnikrishnan et al., 2002). It is

because ZDDP can act as a catalyst to the formation of MoS2 embedded in the short

chain zinc polyphosphate. Moreover, ZDDP play a role to supply excess sulfur

5

provided complete sulfuration of the molybdenum containing compound. The

existence of the layer of MoS2 under the surface of the molybdenum and ZDDP is

believed as the factor the decreasing of coefficient of friction of sliding metal

surfaces. Other than that, if molybdenum compound cannot adsorbs on the metal

surfaces faster than ZDDP. ZDDP will dominate the near surface region and causes

the increasing the coefficient of friction but the compound has become more stable.

The effect of molybdenum compound and ZDDP on friction and wear behaviour

depends on which of them are faster to be adsorbs on the sliding surface

(Komvopoulos and Pernama, 2006). From the result, new research ought to be done

to determine the desirable concentration of both additives as to develop a new bio-

lubricant. Throughout the study, the information on the blending of the molybdenum

and ZDDP are scarce due to lack of research based on the molybdenum organic

metallic compound. Therefore, this study is mainly about the research to determine

desirable concentration of additives by addition of MoDTP into ZDDP induced corn

oil bio-lubricant as tribological properties improver.

1.3 Objective of Research

Based on the problem statement discussed above, the objectives of this study are

listed below:

1. To develop a new biodegradable lubricant with the addition of tribological

property improver additive.

2. To test and characterize the newly developed bio-lubricant.

6

1.4 Scope of Research

In order to achieve the objectives, the scopes are prepared as shown below:

1. Developing new bio-lubricant oil using commercialized cooking corn oil with

addition of MoDTP into ZDDP induced corn oil bio-lubricant as tribological

property improver additive agent.

2. Testing newly developed bio-lubricant oil using RDE-AES (ASTM D6595).

3. Characterizing of newly developed bio-lubricant using four ball testers

(ASTM D4172).

7

CHAPTER 2

LITERATURE REVIEW

2.1 Lubrication

Lubrication is used to minimize the wear and friction of the relative motion

surfaces by using the ability of oil or another liquid (Rudnick, 2009). A study by

Mobarak et al., (2014) stated that a protective film layer was formed by lubricant

between two moving surfaces and it is the reason how friction and wear were

reduced. The primary objective of lubrication despite of reducing wear and friction

is to reduce the normal and shear stress in solid surface contact (Ludema, 1996). On

the other perspective, according to (Hamrock and Schmid, 2004) stated that the

primary purpose of lubricant is to control wear and friction and the secondary

properties of lubricant are listed below:

1. Lubricant can be drawn between the moving parts by hydraulic action.

2. Lubricant has the ability of high heat-sink capacity which is to cool

the contacting parts.

3. Lubricant easily reacting or mixing with chemical for variety

properties of corrosion resistance, protective layer or detergency.

4. Efficiently to remove wear particles.

A study by Torbacke et al. (2014) showed that lubricant has many functions

in tribological contacts. Lubricant played the main role to transfer heat away from

the contact due to presence of friction causes by the contacting of two or more

surfaces has raised the temperature. Without lubricant, the temperature could be

rising high enough to melting the material. Furthermore, contaminant will

accumulate in engine during operation and producing wear debris, sludges, acids,

soot particles or peroxides. The important function of lubricant is to prevent the

8

action of contaminant from damaging the engine (Nehal and Nassar, 2011). Effective

lubricants have an excellent viscosity to maintain the lubricating film under any

operating conditions. Other than that, it should be in liquid state as possible to

remove heat and prevent power loss because of viscous drag (Hamrock and Schmid,

2004). Besides, the application like hydraulic systems was used lubricant to transfer

the power from one point to another (Torbacke et al., 2014).

2.2 Classification of Lubricant

Lubrication has the limitation in technologies and economic because of the

physical and chemical degradation due to factors such as temperature and acids,

vacuum, radiation and weightlessness (Ludema, 1996). The physical appearance of

lubricant influences the application of lubricants in certain operation. Lubricant has

been identified existed in three physical appearances and been use in different

application due the characteristic and potential.

2.2.1 Solid Lubricant

Solid lubricant is any solid materials that are used to reduce friction and

mechanical interactions between relative motions of surfaces against the action of

applied load (Rudnick, 2009). It is a form of powder or thin solid film to provide

protection between two moving surfaces for reducing wear and friction (Bhushan,

2013). It has been introduced after the fall of performance of traditional liquid

lubricants additive (Rudnick, 2009). Moreover, they have been used in small amount

in 1800s and further research were started from 1950 to 1965 for the investigation on

loose powders, metals, tungstates, oxides and molybdates and layer-lattice salts by

the aerospace industry (Ludema, 1996). Solid lubricants usually used when there is

problem to keep liquid lubricant in the contact in some applications (Torbacke et al.,

2014).

9

Solid lubricants are typically as a dry film or an additive in liquid to provide

effective lubricant for different types of applications (Rudnick, 2009). Besides, it

also being used in application involving sliding contact such as operating bearing at

high loads and low speeds (Bhushan, 2013). Furthermore, another application that

used solid lubricant is when the surfaces are not chemically active with lubricant

additives such as polymers or ceramics (Rudnick, 2009). The most popular of solid

lubricants are graphite and molybdenum disulfide. Figure 2.1 displays the difference

between liquid lubricant and solid lubricant.

Figure 2.1: Differences between liquid lubricant and solid lubricant (Rudnick, 2009).

Graphite is a form of carbon and it is one of solid lubricant materials that can

act as lubricant. It has the same characteristic as molybdenum disulfide which is the

decomposition of sheets in hexagonal array. The strong bonding in the sheet of

graphite with the weak bonding of van der Waals between sheets has provided low

shear strength between sheets. Molybdenum disulfide usually used in powder form

because it has possibility to electroplate the surface with molybdenum then treats

with sulphur containing gas to gain bonded molybdenum disulfide (Ludema,1996).

Graphite and molybdenum disulfide can be useable in high temperature and

oxidation atmosphere environments while the liquid lubricants are opposed. In the

condition of extreme temperature and extreme contact pressure, graphite and

molybdenum disulfide is the great choice to be used as lubricant. Other than graphite

and molybdenum disulfide, another compound that can be useful for solid lubricants

are boron nitride, talc, calcium fluoride, cerium fluoride, polytetrafluoroethylene

(PTFE) and tungsten disulfide (Rudnick, 2009).